Transistors, such as IGFETs (insulated gate field effect transistors), which include MOSFETs and IGBTs, are widely used as electronic switches in different kinds of applications, such as inverters, voltage regulators, current regulators, or drive circuits for driving electric loads, such as lamps, valves, motors, etc. Transistors that are commonly employed as power transistors include a plurality of identical transistors cells arranged in a transistor cell field and electrically connected in parallel.
In many modern power transistors, vertical field plates taking advantage of the “charge compensation principle” are used for achieving a low on-resistance (RON) of the transistor. In the charge “compensation principle,” field plates, which are electrically connected to a source zone or to an emitter zone of the transistor, extend into the drift zone of the transistor in order to compensate charges provided by dopants that cause the type of conductivity (n or p) of the drift zone. However, the field plates result in an increase of the output capacitance of such transistors. As a consequence thereof, switching the transistor alternately on and off leads to undesired over-voltage peaks caused by unavoidable inductances of an electronic circuit to which the transistor is connected. As the height of the over-voltage peaks increases with the slew rate of the electric current through the transistor, conventional transistors seek to reduce the slew rate using a damping resistor connected in series with the field plates, which, in view of the required high ampacity of that resistor, wastes a lot of chip space. Further, the switching behaviour of the transistor cells of such a transistor is inhomogeneous, that is, the transistor cells do not switch simultaneously on and off.
Therefore, there is a need for a transistor with a low on-resistance, a low output capacity and a homogeneous switching behaviour.